The invention relates generally to telecommunication systems. More precisely, the invention relates to a digital cellular radio system utilizing a soft hand-off, particularly to a system implemented with the SDMA method.
A typical cellular radio system comprises a fixed base station network, and a subscriber terminal communicates with one or more base stations of the network. A base station forwards the communication that arrives from a subscriber terminal. While moving or remaining stationary, subscriber terminals may transmit messages via the base stations both to each other and to subscriber terminals of other telephone systems. The transmission of messages is possible when the subscriber terminals are located in the coverage area of the base station network. In order for a subscriber terminal to be able to use the services provided by the cellular radio system, it should maintain a connection to at least one base station under all circumstances. When a subscriber terminal does not use the services provided by the base station network, it does not need a connection to the base station network but it listens to the base stations in an idle mode. When the subscriber terminal moves in the base station network from the coverage area of one base station to the coverage area of another base station, this creates a need to change the channel or the base station.
When a subscriber terminal moves from one cell to another in a cellular radio system, the system performs a hand-off, which is either a hard hand-off in a simple case or a more flexible soft hand-off. In a typical cellular radio system, a subscriber terminal communicates with only one base station at a time, even though especially for example in a CDMA (Code Division Multiple Access) system the subscriber terminal may also communicate simultaneously with several base stations. In a prior art soft hand-off, the connection to the base station network is maintained despite the hand-off. In such a hand-off, the base station is usually changed. The prior art also comprises a softer hand-off where the base station is not changed, but the sector of the base station used is changed. A soft and a softer hand-off are called make-before-break-type hand-offs, which means that a new connection is set up for the subscriber terminal before the connection to the previous base station is terminated. The frequency band used is not changed in either hand-off.
Another method is a SDMA (Space Division Multiple Access) method wherein the users are distinguished from one another on the basis of their location. This takes place in such a manner that at the base station the antenna beams are oriented to the desired directions on the basis of the location of the mobile stations. This requires the use of adaptive antenna arrays, i.e. phased antennas, and processing of the received signal by means of which the mobile stations are tracked.
The use of SDMA in connection with CDMA provides several advantages compared to the prior methods, such as sectorization. If the beams of the sectors are narrowed in sectorization in order to increase the spectral efficiency, the number of the hand-offs to be carried out from one sector to another also increases. This in turn increases too much the calculation capacity required in the base station controller, for example.
Prior hand-off methods have drawbacks, however. All the base stations participating in the hand-off do not transmit a signal to the subscriber unit rapidly enough. The aforementioned problems also occur in the reception of signals transmitted by the subscriber terminal. Problems occur, for example, since the transceiver antennas of the base stations cannot be oriented sufficiently rapidly to the optimum direction, whereupon the direction of the transmission beam of the antenna is not optimum. For example, if the base station does not transmit a signal to the optimum direction, the subscriber terminal does not necessarily receive all the signals that have been transmitted. There have been efforts to eliminate the problems resulting from the orientation of the transmission beams by increasing the transmit power. However, increasing the transmit power has raised the level of interference in the network.
The purpose of the present invention is to realize a method that is used in a cellular radio system which is implemented with the SDMA technique and wherein a base station antenna is oriented rapidly to the optimum direction.
This is achieved with a method according to the invention for setting up a connection, used in a cellular radio system comprising subscriber terminals and base stations that form antenna beams which are oriented towards a subscriber terminal in order to set up an optimum connection, in which method the location of the subscriber terminal in the cellular radio system is calculated, and wherein the base station orients its antenna beam towards the subscriber terminal on the basis of the subscriber terminal location data.
The invention also relates to a method for setting up a connection, used during a hand-off in a cellular radio system comprising at least one subscriber terminal and base stations that form antenna beams which are oriented towards the subscriber terminal in order to set up an optimum connection, in which method the location of the subscriber terminal is calculated on the basis of the connection to the base station, and wherein the antenna beam of the base station that sets up the connection to the subscriber terminal is oriented towards the subscriber terminal by utilizing the location data calculated above.
The invention further relates to a cellular radio system used during a hand-off, the cellular radio system comprising subscriber terminals and base stations that communicate with the subscriber terminals and that form antenna beams that are oriented towards a subscriber terminal in order to set up an optimum connection with the subscriber terminal, the cellular radio system comprising calculation means for calculating the location of the subscriber terminal in the cellular radio system on the basis of the established connection, the base stations that set up the connection to the subscriber terminal orienting their antenna beams towards the subscriber terminal on the basis of the aforementioned location data.
The arrangement according to the invention provides several advantages. The arrangement according to the invention can be used both in the downlink and the uplink direction during a hard and a soft hand-off. The arrangement enables the orientation of the base station antenna rapidly towards the subscriber terminal when the base station is transmitting a signal. The rapid orientation of the antenna enables fast connection set-up during a hand-off. The arrangement also enables a higher network capacity. The antenna is also oriented rapidly towards the subscriber terminal when the base station receives a signal from the subscriber terminal. In the cellular radio system according to the invention, it is possible to use a longer distance between the base station and the subscriber terminal compared to the prior art, since the present arrangement enables a lower interference level. The base station orients its antenna towards the subscriber terminal on the basis of subscriber terminal location data calculated in advance. The arrangement can be used for example in TDMA and CDMA cellular radio systems implemented with the SDMA method.